1. Field of the Invention
The present invention relates to tubular electrical machines, and in particular to tubular electrical machines that are suitable for use either as direct drive generators or linear motors.
2. Description of the Related Art
It is well known to use linear electrical machines either as generators, to convert an input of linear, usually reciprocating, motion into electrical power, or as motors, to produce a linear movement from an electrical power source. Most linear machines use a flat arrangement, which is based upon the workings of a conventional rotating machine that has been split open to form a flat surface. One of the disadvantages of this arrangement is that the mechanical support of the moving and flat parts can be quite complex. Another disadvantage is that, like rotating machines, flat linear machines usually contain end-windings that do not contribute to the electro-mechanical power conversion process.
Tubular electrical machines are also known and operate in substantially the same manner as these linear machines. They can be considered to be linear machines that have been wrapped up such that the coils in what was previously the flat part of the linear machine become circular and therefore contain no end-windings. The tubular structure has the added benefits that the machines are inherently strong and their mechanical support is a lot less complex than that of conventional linear machines. Permanent magnet-based tubular machines may be formed in one of two ways. The circular coils of the armature winding may be formed in slots provided in a substantially cylindrical outer surface of a radially inner member, which is surrounded by a radially outer tubular member having a substantially cylindrical inner surface that contains rows of permanent magnets. A tubular machine using permanent magnets in this way is described in U.S. Pat. No. 6,787,944. Alternatively, the rows of permanent magnets may be situated on the substantially cylindrical outer surface of the radially inner member and the coils of the armature winding may be formed in slots in the substantially cylindrical inner surface of the radially outer member. In both cases it is usual that, when the tubular machine is in use, the outer member is held stationary and the inner member moves reciprocally relative to it. The opposite case is also possible but is generally less common as it is harder to mechanically support a tubular machine that operates in this manner. Tubular machines may also be of synchronous solid salient pole with wound field coils, induction or reluctance type and these may be formed in substantially the same manner as those tubular machines that use permanent magnets.
There are several problems with the construction and operation of conventional tubular electrical machines. Firstly, the component containing the armature winding (which can be the radially outer member or the radially inner member depending on the particular construction of the tubular machine) is usually formed of a magnetic material such as iron. In large scale tubular machines, like those designed for converting wave energy into electrical energy, the formation of eddy currents in the magnetic component is a major problem that can only be addressed by using expensive and sometimes ineffective amorphous magnetic materials, or expensive manufacturing processes. Designs for permanent magnet tubular machines without magnetic material have been proposed but currently these designs have low power factors and low efficiency.
Another issue is the peak to mean power ratio of existing permanent magnet tubular machines. Because of various electromagnetic limitations in these machines their peak to mean power ratio is typically less than 3:2. This means that the tubular machine has to be electromagnetically designed for almost peak power and there is no significant short-term overload capacity. This is a particular problem in applications such as the generation of electricity from wave power, where the peak to average power is typically very high.
Several designs for superconducting rotating machines have been proposed. The current design of large superconducting rotating machines is dominated by the conventional synchronous machine arrangement with superconducting field windings and conventional or non-superconducting armature windings as disclosed in European Patent Application 1247325. Such superconducting synchronous rotating machines can be made considerably smaller than conventional synchronous rotating machines of the same power rating. This is a result of the very high current density and consequently high flux density that can be achieved by superconducting field windings.